Multiphase Growth and Electronic Structure of Ultrathin

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J. Phys. Chem. C 2007, 111, 10493-10497

10493

Multiphase Growth and Electronic Structure of Ultrathin Hexaazatrinaphthylene on Au(111) Sieu D. Ha,*,† Bilal R. Kaafarani,‡,§ Stephen Barlow,‡ Seth R. Marder,‡ and Antoine Kahn† Department of Electrical Engineering, Princeton UniVersity, Princeton, New Jersey 08544, and School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: March 6, 2007; In Final Form: May 7, 2007

Scanning tunneling microscopy was used to study the morphological behavior of 1-2 monolayers (ML) of 5,6,11,12,17,18-hexaazatrinaphthylene (HATNA) deposited on Au(111). The first ML consisted of molecules flat on the surface, arranged in one of three different ordered phases depending on annealing parameters. The second ML assumed an upright orientation and tended to order only if the first layer was disordered. Scanning tunneling spectroscopy of 1-2 ML ordered HATNA was compared to ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy of 80 Å amorphous HATNA. There is good agreement between techniques in the transport band gap (∼3.80 eV), but the position of the Fermi level in the gap is shifted in the tunneling spectra by about 0.75 eV toward the highest occupied molecular orbital. The shift may be due to morphological effects or degradation from ultraviolet light irradiation.

Introduction Organic electronics research has progressed significantly in the past two decades toward competitive consumer applications. Products are beginning to enter the market that utilize the advantages of organic thin film technology in novel displays, low-cost electronics, and other devices outside the realm of inorganic semiconductors. Yet, there continues to be challenging obstacles that limit device performance. Among such problems is low charge carrier mobility in electron transport materials (ETMs). In response to these needs is recent research in new classes of materials, among which is the family of discoid molecules based on 5,6,11,12,17,18-hexaazatrinaphthylene (HATNA), shown in Figure 1 (inset). Substituted derivatives have been developed that are columnar liquid crystals.1,2 These 1-D columnar structures enhance π-orbital overlap and promote carrier hopping between the cores along the stacking direction. Mobilities as high as 0.3 cm2/V s have been reported for 2,3,8,9,14,15-(hexaalkylsulfanyl)-functionalized HATNA derivatives in their liquid crystalline states, with a higher mobility of 0.9 cm2/V s being reported in the columnar solid phase.1 Even in noncrystalline films of HATNA derivatives, mobility (assumed to be electron mobility from energetic considerations) can reach 0.02 cm2/V s, which is remarkably high for an amorphous organic film.3 This marks significant improvement over typical ETMs such as Alq3, which has an electron mobility of 10-6 cm2/V s.4 Furthermore, electron spectroscopy has shown that the electron affinity of some HATNA derivatives is relatively large as compared to other standard ETMs, ensuring more facile electron injection.5 To date, there have not been any reports of scanning tunneling microscopy (STM) studies of HATNA or its derivatives. STM * Corresponding author. E-mail: [email protected]; tel.: (609) 2582056. † Princeton University. ‡ Georgia Institute of Technology. § Current address: Department of Chemistry, American University of Beirut, Beirut 1107-2020, Lebanon.

Figure 1. STM image of 1 ML 5,6,11,12,17,18-hexaazatrinaphthylene (HATNA)/Au(111), showing close resemblance between the imaged molecules and the chemical model (35.0 nm × 35.0 nm; Vsample ) -1.7 V, and I ) 70 pA). Inset: chemical model of HATNA.

has, however, recently been used to study a monolayer (ML) of a derivative of the related 1,4,5,8,9,12-hexaazatriphenylene (HAT) on HOPG.6 In this system, the molecular structure is strongly influenced by hydrogen bonding between substituents, and the molecules lie flat on the substrate. As for HATNA, X-ray diffraction methods have established the single-crystal structure of two solvates7,8 and a hexachloro derivative,5 and they have been used to provide insight into the packing of hexa(alkylsulfanyl) derivatives.1 However, it is not known how HATNA and its derivatives order on a metallic substrate, if at all. To that end, the results of an STM investigation of 1-2

10.1021/jp0718404 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/27/2007

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ML HATNA/Au(111) are presented here. Au(111) was chosen as a substrate due to its known ease of preparation on mica, on which it forms atomically flat planes, and for its low reactivity. Following the STM discussion are results from scanning tunneling spectroscopy (STS) studies, which were performed on the same films as STM to establish how the substrate affects organic adsorbant energy levels at the interface and to compare with ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) of occupied and unoccupied states on thick films. Experimental Procedures Au(111)/mica substrates were purchased hydrogen flameannealed from Molecular Imaging. Prior to organic deposition, the substrates were cleaned by repeated cycles of Ar+ sputtering (0.5 kV, 2.0 × 10-5 Torr) and annealing (400 °C), which produced clean surfaces with terraces typically about 100 nm wide, exhibiting the standard 22 × x3 reconstruction of Au(111) as observed by STM. HATNA molecules were synthesized as previously described.5 HATNA was deposited onto Au(111)/ mica by thermal evaporation from a quartz crucible in ultrahigh vacuum (UHV, 10-10 Torr) at about 1 Å/min with the substrate at room temperature. Subsequent substrate annealing facilitated molecular ordering, as described in the next section. STM was performed in constant-current mode with an Omicron room temperature STM in UHV (5 × 10-11 Torr). Tungsten STM tips were electrochemically etched from W wire in a 2 M NaOH solution using a tip-etch circuit.9 They were loaded into UHV and electron-bombarded in situ to remove the oxide layer. STM images were processed with WSxM software from Nanotec Electronica.10 Electron spectroscopy measurements of occupied and unoccupied states of thick HATNA films were carried out in a separate UHV system equipped with UPS and IPES. The substrates used in these experiments were not crystalline Au(111)/mica but rather polycrystalline Au (1500 Å)/Ti (100 Å)/n+-Si(100) deposited by e-beam evaporation. HATNA films 50-150 Å thick were deposited in a similar fashion as for the STM samples, by thermal evaporation from a quartz crucible. The transport gap measured by UPS/IPES on thick films was compared with results from STS on molecularly ordered thin films. Results and Discussion First ML. HATNA films are amorphous when deposited on Au(111) at room temperature. They exhibit no observable order in STM, and there is no appreciable surface diffusion at room temperature. However, upon annealing, the first ML of HATNA orders into one of three phases depending on the parameters used. Samples were annealed by slowly ramping up the substrate temperature to 100-125 °C over a 15-20 min interval, but they were not held long at elevated temperature because of the concern of desorbing the molecules altogether. STM images were recorded after the samples cooled to room temperature, and since room temperature surface diffusion is negligible, any observed phase has the molecules in an equilibrium state. After annealing, STM reveals that the first ML of HATNA molecules lies flat on the surface of Au(111). Figure 1 shows a typical STM image of the most energetically stable configuration. The molecules order consistently on 100 nm wide terraces and remain in a stable configuration during scanning regardless of phase. Moreover, STM images of individual HATNA molecules closely resemble the expected three-spoke shape of the chemical structure. At negative sample bias, the molecules appear mostly uniform in contrast, indicating sig-

Figure 2. (a-c) STM images of different phases of 1 ML HATNA/ Au(111) in order of increasing energetic stability (10.0 nm × 10.0 nm; Vsample ) -1.8 V, and I ) 100 pA except for panel a, where I ) 150 pA); unit cells are illustrated.

nificant delocalization of occupied electronic states throughout the molecule. The clear vertical zig-zag lines seen in Figure 1 are due to the Au(111) reconstruction and are discussed next. The three phases of 1 ML HATNA on Au(111) are shown in order of increasing energetic stability in Figure 2a-c. The least stable phase is observed after annealing to 100 °C. Here, molecules are organized into rows of about 15.0 Å width, and within each row, adjacent molecules have opposite orientations. Modulo the molecular rotation, the structure is approximately hexagonal close-packed (HCP), forming a 19.1 Å × 17.0 Å unit cell with two molecules per cell for a density of 0.65 molecules/nm2. In Figure 2a, the image is slightly distorted due to sample drift, but the calculations are from undistorted images and are accurate. After annealing several more times to 100 °C (45 min total), the next favorable phase is observed. In this case, all molecules are parallel in an HCP structure. There is one molecule per 13.4 Å × 12.9 Å unit cell, which equates to a packing density of 0.73 molecules/nm2. Last, the most energetically favorable phase, achieved by annealing to 125 °C several times (90 min total), is also the most structurally complex. In this ordering, molecules assume one of four orientations. In each row, molecules form a herringbone pattern with two of the four orientations, and adjacent rows alternate which two constitute the herringbone. The net effect is a 25.0 Å × 22.9 Å rectangular unit cell with four molecules per cell, resulting in 0.71 molecules/nm2 packing density. Note that at a higher packing density, there must be a greater number of molecules in the first layer for full ML coverage. Indeed, in images of the least energetically favorable phase, there are often disordered multilayer clusters of molecules that, upon annealing, likely decrease in size and contribute to the number of molecules in the first ML. There are still such clusters in the intermediate and most favorable phases, but they are smaller and appear much less frequently. The proposed rank of energetic stability comes from several considerations. By annealing to increasingly higher temperatures as described, the molecular order of HATNA/Au(111) can be reproducibly and predictably altered. The film goes from being

Ultrathin Hexaazatrinaphthylene on Au(111) amorphous through three phase transitions to the alternating herringbone structure. There are also instances in which two phases are observed concomitantly (e.g., the intermediate phase can be observed on the same surface as either the most or the least stable phase, but the same cannot be said for the latter two). Moreover, annealing to higher temperatures (∼150 °C) leads to molecular desorption, which leaves bare Au(111) and indicates that no additional phases can be accessed that are more energetically stable than those specified. Finally, the estimated density of each phase roughly agrees with the proposed sequence. As the density increases, the surface free energy decreases, and the film becomes more stable and thus more energetically favorable. Note that although the intermediate phase is slightly more dense than the most favorable phase, it is still less favorable, likely because of steric hindrance between molecules as suggested by Monte Carlo simulations on tripodshaped molecules.11 Note that regardless of the molecular ordering, the substrate reconstruction lines typically seen on bare Au(111) are still visible with 1 ML of adsorbed molecules, as is the case with other small molecules deposited on Au(111).12,13 These lines are due to alternating domains of FCC and HCP structure at the metal surface. The reconstruction visibility through the organic layer indicates that the metal substrate affects and contributes to the spatial distribution of electronic states in the first ML. It also suggests that the molecule-substrate interaction is relatively weak as the molecules do not significantly perturb the metal structure. This is further supported by the low desorption temperature of the first ML. In contrast, other small molecules such as hexa-peri-hexabenzocoronene (HBC) strongly interact with Au(111) as evidenced by their high desorption temperature (>600 °C for HBC) and their noticeable alteration of metal reconstruction features.14 Second ML. Initial attempts to produce ordered multilayer HATNA/Au(111) films were not successful. Overlayers deposited onto an ordered ML are amorphous and highly sensitive to annealing temperature. Changing the temperature by just a few degrees Celsius transforms the result from molecular disorder to desorption. Even using lower annealing temperatures for extended time periods does not improve the structure. However, depositing a thick (∼15-20 Å) amorphous film onto bare Au(111) followed by annealing to about 100 °C produces an ordered 2 ML surface. STM topographic images of such a system are shown in Figure 3a,b. The large scale image in Figure 3a shows an area of the substrate from which most of the second ML has desorbed, leaving the first ML organized according to the least energetically stable phase with its characteristic rows. The large equilateral triangle in the center corresponds to a monatomic step up of Au(111), with a height of 2.7 Å, in fair agreement with the literature value of 2.4 Å.15 On the righthand side, remnants of second MLs are visible in the bright and irregularly shaped regions. Note that the Au(111) reconstruction features are not obviously apparent in these parts but that they remain visible where there is only 1 ML. This implies that the substrate does not strongly affect the spatial charge distribution beyond the first ML. The apparent step height between the first and second MLs is about 4.5 Å. In this image, there is no indication as to which of the three phases the molecules below the second ML form, if any. At the interface between MLs, the image appears noisy, which implies molecular motion due to diffusion and disorder. It also suggests that the molecules below the second ML are not well-ordered. What likely occurs during annealing of a thick amorphous HATNA film is that the topmost layer orders before the interface

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Figure 3. STM images of 2 ML HATNA/Au(111) showing (a) first and second ML in the same image (80.0 nm × 80.0 nm; Vsample ) -1.8 V, and I ) 80 pA) and (b) an enlarged image of the second ML herringbone structure (15.0 nm × 15.0 nm; Vsample ) -1.8 V, and I ) 60 pA). (c) Perspective view of proposed model of second ML phase with four upright molecules forming two rod-like units.

10496 J. Phys. Chem. C, Vol. 111, No. 28, 2007 layer. Then, the overlayer desorbs, leaving a disordered singleML film, which will then order in the least favorable phase upon further annealing, as observed here. Because of the energetics of defect sites and step edges that may enhance or retard this process, all three stages of ordering, desorption, and reordering may coexist on one surface. Figure 3b is a magnified view of the second ML of HATNA molecules. Here, it is evident that the second ML is organized into a herringbone structure of rod-like shapes. The rods are about 27 Å × 7.5 Å and unlike the molecules in the first ML, they do not physically resemble the expected chemical model in any fashion. The unit cell is 25.9 Å × 25.9 Å with two rods per cell for a density of 0.30 rods/nm2. Upon closer inspection, each rod actually appears to be composed of two halves, with the outer portion of each half brighter than the inner portion. Given the dimensions of the rods, it is conceivable that each unit is composed of two HATNA molecules, each standing on one outer lobe such that two lobes per molecule face upward. Figure 3c shows a 3-D perspective view of four upright molecules in this proposed configuration. Moreover, both molecules are either tilted inward or there is an electronic effect that causes the bright spots on the rod edges. While this model still lacks corroborative evidence from computation or alternate structural techniques, there are other molecules such as diindenoperylene,16 sexithiophene,17 and pentacene18 that also grow upright on amorphous surfaces. Regardless of the second ML ordering, it is clear that at the interface with Au(111), nonfunctionalized HATNA molecules do not readily adopt π-stacked structures. This is in contrast to the structures formed by HATNA‚CHCl3,7 the hexachloro derivative,5 and the columnar liquid crystals1 in which extensive π-stacking is observed. However, the structure of HATNA‚4CHCl3 suggests that π-stacking in HATNA can easily be disrupted by other factors.8 The single-crystal structure of HATNA‚4CHCl3 does not feature extended π-stacks but only face-to-face π-interactions between pairs of molecules. STS. Scanning tunneling spectra from the ordered first and second MLs are compared with UPS and IPES spectra in Figure 4a. Ultraviolet photoemission and inverse photoemission experiments were performed on 80 Å amorphous HATNA deposited on polycrystalline gold. In contrast, STS was performed on ultrathin layers of ordered molecules deposited on crystalline Au(111). Nevertheless, the measured bandgaps from both spectroscopic techniques are in reasonable agreement. From UPS and IPES, the ionization potential IP ) 6.58 eV and the electron affinity EA ) 2.76 eV, giving a transport gap of Eg ) 3.82 eV.3 The work function of HATNA/Au Φ ) 4.45 eV, which equates to the LUMO lying 1.69 eV above EF and the HOMO lying 2.13 eV below EF. STS, on the other hand, does not give reliable information about the surface vacuum level, so IP, EA, and Φ are indeterminable, but the positions of the HOMO and LUMO with respect to EF can be deduced. On the first ML, the LUMO is about 2.41 eV above EF and the HOMO is about 1.29 eV below EF, which gives a band gap of 3.70 eV. On the second ML, the LUMO is about 2.45 eV above EF and the HOMO is about 1.40 eV below EF, which gives a band gap of 3.85 eV. All of the energy levels are summarized in Figure 4b. Within experimental error, the band gap measured on the second ML agrees with that measured by UPS/IPES. The band gap of the first ML is smaller than these due to image charge effects at the metal interface. These effects relax charge carriers and thus decrease the transport gap more than weak molecular polarization in the bulk far from the metal.19

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Figure 4. (a) STS (dI/dV)/(I/V) spectra of 1-2 ML HATNA/Au(111) as compared to UPS and IPES spectra of an 80 Å HATNA film on Au and (b) corresponding energy level diagram.

Despite the band gap agreement, there is a consistent shift of about 0.75 V in the Fermi level position between UPS/IPES and STS in this data, which is not understood well. Several effects may be ruled out as causes, based on UPS experiments. A layer-by-layer study determined that band bending is not responsible for the difference in relative EF positions. There is indeed little difference between the energetics of a thin, amorphous HATNA film and a thick, amorphous HATNA film. Work function is also not a cause, as both forms of gold (Au(111) and polycrystalline Au) have similar work function of 5.27 eV. This leaves just a few possibilities. Morphological differences between ordered HATNA films for STS and unordered HATNA films for UPS/IPES could cause a shift, as similar effects have been observed with CuPc on Si(111).20 There also may be degradation effects from the ultraviolet lamp used for UPS. After just 5-10 min of UV exposure, the irradiated area on the sample appeared blue, while the rest of the sample was pale yellow. This discoloration implies some level of degradation, which may cause the Fermi level to shift with respect to the HOMO/LUMO levels. Conclusion HATNA/Au(111) films were studied with STM and STS. The first ML of HATNA orders into one of three phases depending on annealing temperature. Regardless of phase, the substrate influences the spatial charge distribution in the organic layer, as evidenced by the visibility of the Au(111) reconstruction features. The substrate-molecule interaction is weak because the desorption temperature is relatively low, and the metal reconstruction is unperturbed by HATNA adsorption. The second ML does not tend to order if the first ML is already

Ultrathin Hexaazatrinaphthylene on Au(111) ordered. However, if the first ML is unordered, annealing leads to organization of the second ML into a herringbone structure that looks drastically different than the first ML. Molecular planes in this phase are likely normal to the surface. These results imply that HATNA does not form ordered multilayers or columnar stacks on Au(111) even though π-stacks are common features of other HATNA-based films. STS data confirm previous UPS/IPES data on the magnitude of the HATNA transport gap. It also shows that the band gap of the first ML is smaller than the second, which is expected from polarization effects at the metal interface. There is a discrepancy between electron spectroscopy and tunneling spectroscopy data in the position of the Fermi level within the gap. The origin of this discrepancy is undetermined but may be related to morphological differences in the organic film or degradation from UV light irradiation. Acknowledgment. We acknowledge support from the National Science Foundation (DMR-0408589) and an American Society for Engineering Education NDSEG fellowship. We thank Jaehyung Hwang for performing the UPS experiments and Eric Salomon for numerous discussions. Work at the Georgia Institute of Technology was supported by the National Science Foundation (CHE-0211419 and the STC Program under Agreement DMR-0120967), Lintec Corporation, and the Office of Naval Research (N00014-04-1-0120). References and Notes (1) Lehmann, M.; Kestemont, G.; Aspe, R. G.; Buess-Herman, C.; Koch, M. H. J.; Debije, M. G.; Piris, J.; Haas, M. P. D.; Warman, J. M.; Watson, M. D.; Lemaur, V.; Cornil, J.; Geerts, Y. H.; Gearba, R.; Ivanov, D. A. Chem.sEur. J. 2005, 11, 3349-3362. (2) Crispin, X.; Cornil, J.; Friedlein, R.; Okudaira, K. K.; Lemaur, V.; Crispin, A.; Kestemont, G.; Lehmann, M.; Fahlman, M.; Lazzaroni, R.;

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10497 Geerts, Y.; Wendin, G.; Ueno, N.; Bre´das, J. L.; Salaneck, W. R. J. Am. Chem. Soc. 2004, 126, 11889-11899. (3) Kaafarani, B. R.; Kondo, T.; Yu, J.; Zhang, Q.; Dattilo, D.; Risko, C.; Jones, S. C.; Barlow, S.; Domercq, B.; Amy, F.; Kahn, A.; Bre´das, J. L.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2005, 127, 1635816359. (4) Kepler, R. G.; Beeson, P. M.; Jacobs, S. J.; Anderson, R. A.; Sinclair, M. B.; Valencia, V. S.; Cahill, P. A. Appl. Phys. Lett. 1995, 66, 3618-3620. (5) Barlow, S.; Zhang, Q.; Kaafarani, B. R.; Risko, C.; Amy, F.; Chan, C. K.; Domercq, B.; Starikova, Z. A.; Antipin, M. Y.; Timofeeva, T. V.; Kippelen, B.; Bre´das, J. L.; Kahn, A.; Marder, S. R. Chem.sEur. J. 2007, 13, 3537-3547. (6) Palma, M.; Levin, J.; Lemaur, V.; Liscio, A.; Palermo, V.; Cornil, J.; Geerts, Y.; Lehmann, M.; Samorı`, P. AdV. Mater. 2006, 18, 3313-3317. (7) Du, M.; Bu, X.-H.; Biradha, K. Acta. Crystallogr., Sect. C: Cryst. Struct. Commun. 2001, 57, 199-200. (8) Alfonso, M.; Stoeckli-Evans, H. Acta. Crystallogr., Sect. E: Struct. Rep. Online 2001, 57, 242-244. (9) Ibe, J. P.; Bey, J. P. P.; Brandow, S. L.; Brizzolara, R. A.; Burnham, N. A.; DiLella, D. P.; Lee, K. P.; Marrian, C. R. K.; Colton, R. J. J. Vac. Sci. Technol., A 1990, 8, 3570-3575. (10) Nanotec Electronica. WSxM software; Spain. Free software downloadable at http://www.nanotec.es. (11) Osipov, M. A.; Stelzer, J. Phys. ReV. E 2003, 67, 061707-15. (12) Chizhov, I.; Scoles, G.; Kahn, A. Langmuir 2000, 16, 4358-4361. (13) Mannsfeld, S.; Toerker, M.; Schmitz-Hubsch, T.; Sellam, F.; Fritz, T.; Leo, K. Org. Electron. 2001, 2, 121-134. (14) Proehl, H.; Toerker, M.; Sellam, F.; Fritz, T.; Leo, K.; Simpson, C.; Mu¨llen, K. Phys. ReV. B 2001, 63, 205409-6. (15) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Phys. ReV. B 1990, 42, 9307-9318. (16) Durr, A. C.; Schreiber, F.; Munch, M.; Karl, N.; Krause, B.; Kruppa, V.; Dosch, H. Appl. Phys. Lett. 2002, 81, 2276-2278. (17) Heiner, C. E.; Dreyer, J.; Hertel, I. V.; Koch, N.; Ritze, H. H.; Widdra, W.; Winter, B. Appl. Phys. Lett. 2005, 87, 093501-3. (18) Pratontep, S.; Nuesch, F.; Zuppiroli, L.; Brinkmann, M. Phys. ReV. B 2005, 72, 085211-5. (19) Tsiper, E. V.; Soos, Z. G.; Gao, W.; Kahn, A. Chem. Phys. Lett. 2002, 360, 47-52. (20) Grzadziel, L.; Zak, J.; Szuber, J. Thin Solid Films 2003, 436, 7075.